Doped semiconductors have long been characterized by sandwiching a metal conducting layer, an insulating layer, a layer of the semiconductor to be tested, and another layer of metal. Typically the capacitance is measured as a function of the voltage applied to the metal layers. The functional dependence of measured capacitance to the applied bias voltage can be related to properties of the incorporated semiconductor. A typical property of interest is the dopant type and concentration.
While the above technique is useful for measuring properties of bulk semiconductors, it is not easily applied to the measurement of local properties in, for instance, a fabricated microelectronic circuit.
Atomic force microscopes have been applied to the problem by creating an in situ capacitor by placing a sharp conducting probe in contact with a doped semiconductor sample sitting on a conducting substrate. The tip-sample capacitance is typically measured using a circuit with a high resonance frequency (typically in the 1 GHz range). The resonance frequency of this circuit is a function of the tip-sample capacitance. Typically this frequency is not measured directly, but instead a lower frequency bias voltage oscillation is applied between the tip and the sample concurrently with a high frequency excitation of the circuit. The standard technique ultimately results in a measurement of the variation of tip-sample capacitance with applied bias voltage. This is enough information to identify neighboring regions as P doped or N doped, but not enough to determine the absolute dopant concentration. For most non-semiconducting samples, the above mentioned method will not yield any signal.
One exemplary method based on the techniques and apparatus described herein employs an AFM and a vector network analyzer to directly measure the resonance frequency of a circuit including tip and sample, and thereby to measure the tip-sample capacitance. The method does not require the application of a DC or time varying tip-sample bias voltage (although it does allow the application of voltage biases) and is not limited to semiconductor samples. The method can be applied in a scanning situation to produce an image variation in tip-sample capacitance. It can also be applied at a single point to produce plots of tip-sample capacitance as a function of tip-sample bias voltage.
An AFM is a device used to produce images of surface topography (and/or other sample characteristics) based on information obtained from scanning (e.g., rastering) a sharp probe on the end of a cantilever relative to the surface of the sample. Topographical and/or other features of the surface are detected by detecting changes in deflection and/or oscillation characteristics of the cantilever (e.g., by detecting small changes in deflection, phase, frequency, etc., and using feedback to return the system to a reference state). By scanning the probe relative to the sample, a “map” of the sample topography or other sample characteristics may be obtained.
Changes in deflection or in oscillation of the cantilever are typically detected by an optical lever arrangement whereby a light beam is directed onto the cantilever in the same reference frame as the optical lever. The beam reflected from the cantilever illuminates a position sensitive detector (PSD). As the deflection or oscillation of the cantilever changes, the position of the reflected spot on the PSD changes, causing a change in the output from the PSD. Changes in the deflection or oscillation of the cantilever are typically made to trigger a change in the vertical position of the cantilever base relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the XY plane defined by the sample), in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that is typically used to generate an AFM image.
AFMs can be operated in a number of different sample characterization modes, including contact mode where the tip of the cantilever is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface.